This invention relates to a method of improving the electrical conductivity of a shaped resin article and an electrostatic coating process.
The hitherto-known technology for improving the electrical conductivity of a shaped resin article includes a method which comprises kneading carbon black, carbon fiber, conductive mica or the like into the molding resin.
However, in this prior art method for improving the electrical conductivity of a resin article, no appreciable improvement in electrical conductivity can be expected unless an electrically conductive additive such as carbon black is incorporated in a large amount, with the result that the final resin product is inevitably handicapped in physical properties. Black coloration is another disadvantage restricting the scope of application of the method.
Recently disclosed is a method in which a complex of a polyoxyalkylene polyol with an electrolyte salt soluble in the polyol is kneaded into a resin molding material and, after molding into a shaped article, the surface of the article is subjected to plasma treatment (Japanese Kokai Patent Publication No. 3-101874). However, this method has the disadvantage of low productivity because of its being a batch process, as well as the disadvantage that the load must be subjected to reduced pressure during plasma treatment.
On the other hand, the conventional technology for electrostatic coating of a shaped resin article includes a method which, as described inter alia in Japanese Kokai Patent Publication No. 50-66538, comprises forming an electrically conductive primer layer on the surface of a resin article by, for example, coating said surface with a conductive paint containing an electrically conductive metal powder to impart electrical conductivity to the surface and, then, performing electrostatic coating. Another known method in this field of the art comprises kneading an electrically conductive inorganic substance such as carbon black, carbon fiber, conductive mica or the like into a resin molding material and, after molding, performing electrostatic coating.
However, when an electrically conductive primer layer is deposited on the surface of a shaped resin article, as taught by Japanese Kokai Patent Publication No. 50-66538, only poor adhesion is assured between the resin surface and the primer so that a plurality of different electrically conductive primers must be deposited in a plurality of layers to overcome the disadvantage. This practice is not only disadvantageous with respect to electrical conductivity and productivity, but the use of different kinds of electrically conductive primers leads to coating wastes and increased costs of production.
Furthermore, when electrostatic coating is performed on an article containing an electrically conductive inorganic substance such as carbon black, carbon fiber, conductive mica or the like, the conductive substance must be used in a large amount and this tends to detract from the physical properties of the resin article. Moreover, the resulting coloration of the resin affects the surface color tone of the electrostatically coated article.
More recently disclosed is a method in which a complex of a polyoxyalkylene polyol with a soluble electrolyte salt is kneaded into a resin molding material, the kneaded mixture is molded, and the surface of the article so produced is subjected to plasma treatment and, then, to electrostatic coating (Japanese Kokai Patent Publication No. 3-101875). However, because the plasma treatment is a batch operation, the method is low in productivity. Moreover, the plasma treatment must be performed under reduced pressure.